In digital communications systems using Time Division Multiple Access (TDMA) schemes, to apportion the available spectrum to the communication users, digital receivers are typically designed for operation in the time-domain to recover the digitally transmitted information. As the technology used in such digital communications systems has increased in complexity, the requirement on the computational capabilities of digital radios has increased. Consequently, there is an increasing need for more optimal use of the digital signal processing resource within digital radios.
In a TDMA system data recovery is generally carried out in the time-domain for a whole slot of data using, for example, a Viterbi decoder. The traditional approach to the implementation of an optimal digital receiver uses a matched filter followed by a (infinite optimal or finite sub-optimal) tap delay equaliser operating in the time-domain. The matched filter at the receiver corresponds to the transmit filter of the digital transmitter. In digital systems the transmitted data symbols are convolved with the transmit filter impulse response (g(t)) to provide the transmitted signal, s(t)=g(t)* data. This transmitted signal needs to be demodulated and the original data symbols recovered at the digital receiver.
A time-domain implementation typically uses an equaliser to compensate for any inter-symbol interference (ISI) of the received symbols that has been caused within the channel. This is not necessarily an optimal solution. Moreover, having a complex channel impulse (in the sense of real and imaginary parts), requires a complex finite impulse response (FIR) filter for equalisation purposes, making the solution even more computationally expensive.
Typically, such operations are computationally complex, requiring a significant amount of the signal processing resource of a digital radio. Additionally, in order to accommodate the use of time-domain equalisers, effort is directed at achieving a Nyquist pulse shape response for the transmitted and received symbols.
Thus there is a need for an improved method of recovering symbols of a digitally modulated radio signal which avoids some of the disadvantages of prior art arrangements.